Some 76 people tested positive on Tuesday – the highest single day figure since the start of the pandemic.
The number of requests for testing has also more than tripled in two weeks, from around 40 a day to 140 on Wednesday.
The “significant” recent increase in case numbers has resulted in very high levels of self-isolation across communities, NHS Western Isles said.
The majority of cases are believed to be on Lewis.
Hospitalisation figures remain low, but leaders warned self-isolation is having an escalating impact on services, including within the NHS.
If the situation continues, other local organisations, services and businesses will likely be affected by staff shortages, they said.
NHS Western Isles chief executive Gordon Jamieson said: “The NHS Western Isles Resilience Group has been convened to monitor the current situation and any potential impact on NHS services and to take steps to safeguard services.
"At this point, we have been able to avoid service cancellation or having to postpone procedures/appointments. However, staffing in some areas is now proving to be extremely challenging and our staff are going above and beyond the call of duty to ensure that we are able to provide as full a range of services as possible for as long as possible.
"However, this will not be sustainable should cases continue to increase at the current rate.
“Our communities have worked so hard over the past two years to protect each other and we should all be proud of our efforts. We must continue to work together to take the measures that we can, and are able to, to minimise the spread or transmission and disruption, to protect both health and the local economy.”
NHS Western Isles consultant in public health medicine, Dr Dermot Gorman, said the health board was now seeing evidence of re-infection, and urged locals not to become complacent if they have contracted the virus previously.
The Scottish Government began including reports of reinfections in daily case counts on Tuesday, with the proportion of these across Scotland estimated at around 10 per cent of total cases.
It comes as new figures from the Office for National Statistics estimated the number of people in Scotland living with Long Covid for more than 12 months has increased by 25 per cent since last month, to 50,000.
Charity Chest Heart and Stroke expressed concern at the figures. The charity set up a Long Covid support service last year, jointly funded by the Scottish Government.
Campaigns director Lawrence Cowan said: “Month after month, more people in Scotland are estimated to be living with the debilitating effects of Long Covid for more than a year and they desperately need support.
“Listening to people living with the condition, we know how hard it can be to live with Long Covid. People are not only facing life-changing symptoms, but they are also struggling to be heard and understood. They are exhausted and at their wits end.”